The infra-red (IR) radiation region in electromagnetic spectrum covers radiation with wavelengths between those of visible light and microwaves (750 nm respectively 1 mm). Detection of radiation in the IR region has many applications in both civilian and military purposes.
Infra-red detection methods are divided into the two categories of photonic and thermal working principals. The difference between these methods originates from their physical process and this implies different application areas. Photonic detectors have fast response times, in the order of 10−5 s, and operate when an excitation of charge carriers occurs through IR-absorption. The excitation of the charge carriers provides an electronic signal, which can be read at the output. Although the fast response time of photonic detectors makes them attractive they have to be cooled to cryogenic temperatures in order to reduce noise and obtain a useful signal. This makes them viable exclusively for high cost/high performance applications, where the cost of the detector is of secondary importance.
In contrast, thermal detectors follow the principle where the electrical conductivity of a sensing element changes due to heat generated through IR-absorption in a thermistor structure. Thermal detectors have a response time in the order of 10−3 s, and cannot compete with photonic devices for speed. On the other hand, they are cheap to manufacture, and they do not need to be cooled to cryogenic temperatures to function properly. The thermal detector can be a multilayer structure of SiGe/Si with a performance that is strongly dependent on the individual layers thicknesses. A bolometer designed structure, a device for measuring energy of incident electromagnetic radiation which is particularly accurate in the high-λ of the IR spectra, is usually used when exploiting thermistor materials. The bolometer is manufactured through processing of e.g. multilayer structures, to form individual free-standing pixels. Typical thermistor materials are vanadium oxide and single- or multi-layers of (amorphous, poly-crystalline or single-crystalline) silicon or composite silicon-germanium based structures.